"What if the seller says he sent the package (but didn't)? If it was not insured, there is no proof that a package was sent."

If the buyer paid for insurance, and the seller never sent the package, it's not going to do the seller much good to not be able to produce a receipt for insurance. The buyer has put the responsibility on the seller by paying for insurance. It's when the buyer is offered insurance and declines that I expect the buyer to take some responsibility for any loss.

Again, I haven't actually had any problem, and I will handle it on a case by case basis. "The seller's willingness to take responsibility is subject to the buyer's attitude."

And thanks for trying to remind everyone, John. We are not the enemy.